Field of the Invention
The present invention relates to a wrought aluminium alloy product for machining applications, particularly for free-cutting applications. The invention relates further to the field of precision-turned parts obtained from simple extruded products, essentially of the bar or rod type, made of an AA6xxx aluminium having a chemical composition which is optimized as a function of the suitability for extrusion and precision turning, and which, in particular, lacks elements, which have a low melting point and either are harmful to environment and health, such as lead, or can cause embrittlement of the metallurgical structure of the alloy.
Description of Related Art
Unless otherwise indicated, all the values pertaining to the chemical composition of the alloys are expressed in wt %. Moreover, all the aluminium alloys referred to are named, unless otherwise indicated, using the names defined by the Aluminium Association in the regularly published Registration Record Series.
Precision turning relates to the field of large-series manufacture of parts which are generally rotating parts (screw, bolt, axle, etc.) by removing material from bars or rods made of metal. The latter are generally obtained by extrusion from billets, particularly in the case of aluminium alloys. The parts are thus produced at high production rates in cutting machines with manual or digital control. They are used in various fields, from clock making to medical equipment, to the fields of transport (aeronautics, railroad, car) and industry (electrical, electronic, hydraulic . . . ).
The productivity and the surface roughness as well as the dimensional accuracy of the final part are the principal objectives associated with this type of manufacturing. Machinability can be defined as the relative ease of removing metal from a workpiece to obtain a finished product. As regards aluminium alloys, one of the most important properties related to the machinability is the break up of the formed chips. If the chips do not break up, long chips may be formed, which can lead to a wide range of problems, ranging from obtaining products that are out of specification to evacuating chips from the machined piece with high difficulty.
Until recent past, elements such as lead, tin, indium and bismuth were added because they form phases, which act as efficient chip breakers. A typical alloy for free-machining applications is AA6262, which contains a high level of lead and bismuth. Elements such as lead and bismuth, due to their low solubility in aluminium and their low melting point, melt as a result of the heating caused by the machining operation and consequently they form soft areas in the aluminium matrix. Due to soft areas in a hard matrix, chips of small size are easily broken during the machining or precision turning operation, thus allowing a rapid removal of the material and consequently a high machining productivity, preventing also a potential degradation of the final surface roughness of the part because of a higher amount of heat being evacuated with the broken chips.
However, due to the toxicity problems connected with the presence of lead, European laws increasingly limit the admissible content in alloys, such as aluminium alloys, particularly in alloys intended to be used for precision turning. The recent regulated limitation limits the lead concentration of aluminium alloys to 0.4%. For several years, precision turning alloy types were proposed having a low lead content, even being lead-free. Their composition was based on the presence of substitute elements which also have a low melting point, such as, tin, bismuth or indium. These alloys however do not exhibit exactly the same performances during precision turning as the alloys that contain lead. Moreover, these alloys sometimes pose problems of embrittlement due to the complete wetting of the grain boundaries by the low melting point phases originating from the substitute elements. This embrittlement could occur during machining when insufficient cooling is applied.
International patent application WO2005/100623 discloses a free-machining wrought AlMgSi-alloy product, preferably in extruded form, for machining or free-cutting applications, which contains, in weight %: Si 0.6-2.0, Fe 0.2-1.0, Mg 0.5-2.0, Cu max 1.0, Mn max 1.5, Zn max 1.0, Cr max 0.35, Ti max 0.35, Zr 0.04-0.3, impurities max 0.05 each, total max 0.15, Al balance.
Japanese patent application JP9249931 discloses an aluminium alloy with a composition consisting of, by mass, 1.5-12.0% Si, 0.5-6.0% Mg, 0.01-0.1% Ti, and the balance Al with inevitable impurities and containing, if necessary, either or both of 0.5-2.0%, by mass, Mn and 0.1-1.0% Cu or one or more kinds among 0.5-1.0% Fe, 0.1-0.5% Cr and 0.1-0.5% Zr.
U.S. Pat. No. 6,059,902 discloses aluminium alloy extruded products containing Si: 1.5-12%, Mg: 0.5-6% and, optionally, at least one of Mn: 0.5-2%, Cu: 0.15-3% and Cr: 0.04-0.35% and, further, containing Ti: 0.01-0.1%, inevitable impurities, and the balance of Al, in which the average grain size of secondary phase hard particles of Si-system compounds is from 2 to 20 μm and an area ratio thereof is from 2 to 12%. The alloy is melted to obtain a cast billet having DAS (Dendrite Arm Spacing) of 10 to 50 μm, which is then put to a soaking treatment at 450 to 520° C. and then to the extrusion process.
International patent application WO2010/112698 discloses free-machining extruded products made of an aluminium alloy having the following chemical composition, expressed in wt %: 0.8≤Si<1.5%; 1.0<Fe≤1.8%; Cu: <0.1%; Mn: <1%; Mg: 0.6-1.2%; Ni: <3.0%; Cr: <0.25%; Ti:<0.1%; other elements <0.05% each and 0.15% in total, and the balance aluminium.
However, these recently developed alloys do not exhibit in precision-turning applications results as good as the previous lead-containing alloys, in particular in terms of chip breaking.
Therefore, it is still an objective to obtain a free-Pb free-machining wrought alloy product which has properties similar to those products made of lead-containing alloy AA6262 or AA2011, i.e. which prevents from forming long chips during free-machining, has appropriate mechanical properties and corrosion resistance and is suitable to be anodized.